Adding a 2nd Tank for Seahorses
 

I have a 50 Gallon Mix Reef Tank, and would like to add some Seahorses in a separate tank next to my existing setup.

I figured I can the 2nd tank with my same sump setup.

I am new to seahorses. Looking to add a tank anywhere from 20-40 gallons or so.

Is all that I am looking at basically getting 2 seahorses max with this size tank?

What fish work well with seahorses and this size setup if any?

Do the SH need or should i put live rock / sand / inverts?

what do I put in the tank for them to grab onto?

I did exactly what you are describing. I had the tank set up like that for a year. I took it down because the over the top maintenance needed in both tanks to keep the seahorses healthy was too labor intensive. Seahorses are very vulnerable to bacterial infections. Any pockets of detritus or excess food in either tank can cause the seahorses to get sick. Also seahorses need bigger WCs than other fish but with the tanks being connected, both tanks need bigger WCs. You will be better off to set up a totally separate specie specific seahorse tank.

https://farm5.staticflickr.com/4553/...8a2638463e.jpg2017-11-14_10-05-59 by Dawn Gilson, on Flickr
Here is a link to my 2 tanks hooked into the same sump. Actually I raised 15 fry so I had a 3rd 10 gallon tank hooked into the same system.

thanks, which one of these tanks was the SH tank?

I was just thinking 2 of them.

The seahorse tank is the 30 gallon XH which is the one on the left. Originally I only set up the 2 tanks but then my pair of seahorses had babies. For a long time I cared for the fry in a half gallon plastic fish bowl set up as a kreisel nursery that I placed in the sump. However as they outgrew that and were weaned onto frozen food I added the 10 gallon. I was doing insane water changes to keep up with it all.

IMO, it's not worth the risk of potential problems in tying a seahorse tank to a reef tank.
First there is the potential to expose the seahorses to pathogens they haven't been raised with and are VERY susceptible to, but also the seahorse tank should be kept in the range of 68° to 74° F to help mitigate bacterial problems.
The minimum recommended tank size for ONE PAIR of standard sized seahorses is 30g.
As for rock, sand, decor etc, that all depends on the needs/desires of the hobbyist themselves. I use rock as my biological filter with most of it in the sump so I can see the seahorses easier, and, I have bare bottom merely for the ease of cleaning. For decor I use artificial plant replications that don't have higher light needs that many live plants do.
As for adding other fish to the tank, first you would need increased volume, but also have to accept the possibility that pathogen introduction from the other fish may be problematic.
Some people have great success with other fish, but MANY end up with a lot of problems.